Inkjet printers typically use one or more “pens.” In many applications, each pen includes an ink reservoir and a nozzle orifice plate from which ink is discharged. Such pens are typically user-replaceable, having been configured to simply “snap” in or out of the carriage of the inkjet printer.
In many such printers, tolerances between the pen and the carriage, tolerances in the nozzles of the orifice plate and other factors, individually and in combination, direct ink drops in unexpected directions from one or more nozzle openings to the print media. This can result in reduced image quality. However, in many cases compensation may be made for the factors which result in image quality reduction.
In particular, it is known that a “test pattern” or “alignment pattern” may be printed. A sensor may then be used to scan the alignment pattern to gather data. An algorithm may then be used to compare data obtained from scanning the alignment pattern as printed (with possible image quality problems due to pen alignment errors) to theoretical data representing scanning of a correctly printed alignment pattern. Having made the comparison, the algorithm may then calculate a mapping by which input provided to the pens of the printer may be altered to result in the desired output.
A problem is frequently encountered by the sensor when scanning the alignment pattern. In particular, an output of the sensor may have a low signal-to-noise ratio. This problem has been addressed by several proposed solutions. In a first proposed solution, the width of patches of ink contained within the alignment pattern may be increased. The increased width frequently increases the signal-to-noise ratio of the output of the sensor.
A second proposed solution involves selecting LEDs (light emitting diodes) which best illuminate the print alignment pattern during scanning. In particular, LEDs having a spectra (i.e. a frequency of emitted light) that is better suited for use with ink colors used in the print alignment pattern may be selected. Where compatible, the LED color and ink colors combine to increase the signal-to-noise ratio of the output of the sensor.
A third proposed solution is that more than one LED be used to illuminate the alignment pattern as it is scanned by the sensor. Properly balanced, such an LED system can increase the signal-to-noise ratio of the output of the sensor.
Each of the above solutions to the problem of a low signal-to-noise ratio has problems that limit effectiveness and increase cost. A more effective solution to this problem would lower printer cost, increase image quality and provide other advantages.